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Quality Scan: CMM or Gaging?

Art Whistler 

Gaging of some sort is generally used in production lines and cells, in the form of go/no-go or functional checkers, manual bench gages, or inline automatics, to ensure that dimensional integrity is maintained, and to anticipate and manage tool wear or other process variables. The CMM must be evaluated as an alternative to these more traditional means.

There are many compelling reasons to consider the integration of CMMs into production. Above all, the CMM is flexible and capable of measuring almost any parts and features, so there is no need to design and build custom-made tools for the task. Lead times are shorter, and design changes or modifications can be incorporated rather simply via programming rather than by design changes and toolroom work.

The CMM is a strong choice in situations where mixed production would dictate numerous hard gages. If part-holding issues can be resolved efficiently, the CMM is unlimited in the number of programs it can execute. Touch-screen user interfaces have been simplified for unskilled operators, and programs can include digital images and instructions for operation unique to each program. On the other hand, multiple hard gages may each load and function in different ways, requiring extended training and development of "feel" by operators.

Manual hard gages often leave room for operator skill or influence, reducing measurement repeatability. An automatic CMM measures each part at the same location, with the same probing speed, and in a very predictable cycle time, producing excellent repeatability, unless other variables come into play. Anything short of a custom automatic gage will have greater uncertainty in results.   

Some GD&T callouts—for example True Position with Maximum Material Condition, Surface Profile (especially free-form contours), or 3-D relationships—are far more adeptly handled by the CMM computer analysis platform. In some cases it is the only solution.

In the area of acceptance and calibration, one can run into the "clash of cultures." Hard gages normally require the use of setting masters for calibration. Masters might be stock rings or plugs, but more often are expensive part-mimicking custom precision tools needing annual recertification. They can be sized nominally, or a min-max set of two may be required for each gage.

As devices intended for universal applications, CMMs are performance-specified according to ANSI or ISO standards. This is normally acceptable to the CMM user community, typically with annual calibrations, because anything and everything may come through the CMM.The idea of setting masters, per se, is not embraced, and would likely be viewed as nonsense. Manufacturing and production engineers, however, revert to the GR&R tradition: Gage Repeatability & Reproducibility specific to a part. This can be a far more rigorous evaluation than most CMM people realize, because acceptability depends upon more variables than just the CMM performance. The fixture, the operator, and the part-specific inspection program are contributors to additional uncertainty. These must be anticipated and factored into the accuracy calculation.

While the influence of temperature variations in the factory environment has to be considered, it can be said that effects are similar whether one is using manual hard gages or a CMM. The CMM community, however, is culturally attentive to environment, and CMMs today handle this issue with thermal compensation or by design. One does not see many design/build gages that would appear to factor temperature variations into measured results.

Cycle times can vary greatly from simple to complex parts. Custom gages will normally have the advantage in speed of measurements, as CMMs plod around the part acquiring discrete feature data a single point at a time. This is especially true for parts with a few features to be checked. For parts with even minimal complexity, however, the CMM rapidly pulls alongside the fixtures in speed, and blows them away when complex geometry is analyzed.

In the end, it might be reliability that is the deciding factor in choosing one technique over the other. The prevailing perception probably is that CMMs are delicate instruments that belong in a gage room or lab, and are not tough enough for the floor. No doubt a hard gage might fend off a dropped axle where the CMM would be in trouble, but not everyone makes axles—or drops them. New CMM introductions for this segment continue to show greater toughness, smaller size, faster performance, better accuracy, lower cost, and character enough to belong in the production line, and they deserve a hard look.


This article was first published in the August 2008 edition of Manufacturing Engineering magazine. 

Published Date : 8/1/2008

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